Discharge lamps, specifically HID (high-intensity discharge) lamps are used for a large area of applications where energy efficiency and high light intensity are required. Especially in the automotive field, HID lamps are used as vehicle headlamps.
A discharge lamp comprises two electrodes arranged at a distance within a discharge vessel. An arc discharge is generated between the electrodes. Different types of fillings within the discharge vessel are known, distinguishing mercury vapor, metal halide and other types of lamps.
Commercially available lamps for use in a vehicle head-light have an outer bulb which is arranged around the discharge vessel at a distance therefrom. A known type of such a lamp is designed for a nominal power of 35 W and achieves a high efficacy of 80-90 lm/W. After starting such a lamp, a run-up current of, for example, 2.7-3.2 A is necessary, and a run-up power of 75-80 W is used. Thus, the complete HID system comprising lamp, ballast and igniter must be able to operate as these values.
Especially for the automotive field, it would be desirable to have a discharge lamp with lower nominal power, e.g. in the range of 20-30 W, and correspondingly lower demands on the complete HID system. If, however, known lamp designs are simply used at lower power, the lamp efficacy will be dramatically reduced.
US-A-2005/0248278 shows an example of an automotive head lighting discharge lamp with a power of 30 W. The lamp has a ceramic discharge vessel comprising the electrodes, which is surrounded by an outer bulb. The distance between the electrode tips is 5 mm. The discharge vessel has cylindrical shape with an internal diameter of 1.2 mm. The wall thickness of the discharge vessel is 0.4 mm. The discharge vessel comprises a filling which is free from mercury and comprises NaPrI and ZnI2 as well as Xe with a filling pressure of 16 bar. The outer bulb is made of quartz glass and is arranged at a distance of 0.5 mm to the discharge vessel. The outer bulb is filled with N2 with a filling pressure of 1.5 bar at room temperature.
It is an object of the invention to provide a relatively low power HID lamp with high lamp efficacy.
This object is achieved by a high intensity discharge lamp according to claim 1. Dependent claims refer to preferred embodiments of the invention.